How to Detect the Last Item in a Go Template Range?

Detecting the Last Item in a Template Range

In Go templates, iterating over arrays or slices using the range action allows for convenient access to array elements. However, sometimes it becomes necessary to distinguish the last iteration from the others.

Consider a template that requires distinct formatting for the last element in a range:

{{range $i, $e := .SomeField}}
  {{if $i}}, {{end}}
  $e.TheString
{{end}}

This template would generate output like:

one, two, three

To modify this behavior and output "and" before the last element:

one, two, and three

one needs to determine which element in the range is the last.

Although it's possible to track the length of the array outside the template, this method would result in a static value that does not account for changes in the array's length.

The solution lies in leveraging Go's function injection capabilities for templates. By defining a custom function, it becomes feasible to dynamically identify the last element:

package main

import (
    "os"
    "reflect"
    "text/template"
)

var fns = template.FuncMap{
    "last": func(x int, a interface{}) bool {
        return x == reflect.ValueOf(a).Len() - 1
    },
}


func main() {
    t := template.Must(template.New("abc").Funcs(fns).Parse(`{{range  $i, $e := .}}{{if $i}}, {{end}}{{if last $i $}}and {{end}}{{$e}}{{end}}.`))
    a := []string{"one", "two", "three"}
    t.Execute(os.Stdout, a)
}

This approach utilizes reflection to dynamically determine the length of the array and compare it to the current index $i. When the index matches the array length minus one, the last function returns true, indicating that the current element is the last in the range.

As an alternative, one can also use the len function without reflection:

package main

import (
    "os"
    "text/template"
)

func main() {
    t := template.Must(template.New("abc").Parse(`{{range  $i, $e := .}}{{if $i}}, {{end}}{{if len .}}and {{end}}{{$e}}{{end}}.`))
    a := []string{"one", "two", "three"}
    t.Execute(os.Stdout, a)
}

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